Various recombinant and nonrecombinant interferons have been tested in Phase I trials in cancer patients in order to study the toxicity, antitumor effect, immunomodulatory effect and pharmacokinetics of these preparations. The initial Phase I trials employed highly purified recombinant leukocyte A interferon and human Namalva cell lymphoblastoid interferon. These trials, which are now complete, demonstrated that both interferons could be administered safely to patients with disseminated cancer and that the toxicities encountered resembled those previously reported for less purified Cantell alpha interferon. Limited antitumor effect was seen in both trials. Pharmacokinetic analysis reveals dose-dependent levels of serum interferon activity. Immunologic monitoring data indicate unchanged or decreased natural killer cell mediated cytotoxicity, increased monocyte function in a growth inhibition assay and decreased lymphocyte blastogenesis. Other Phase I interferon trials recently initiated include trials of recombinant leukocyte A interferon as an intralesional agent and escalating dose trials of nonrecombinant gamma (immune) interferon and beta (fibroblast) interferon.